El Niño 2026-27: What a Very Strong Winter Does to a Valley Sewer Line
On October 15, NOAA put out its winter outlook for 2026-27 and the National Weather Service director used the words “very strong” for the El Niño building in the Pacific. By the end of October the Climate Prediction Center was giving better than an 83 percent chance of a strong to very strong event from January through March 2027. The outlook favors a wetter than average December through February across the southern tier of the country, California included. NOAA will update it again on November 19.
For a homeowner in Reseda or Granada Hills, that forecast is mostly about one buried pipe: the sewer lateral running from the house out to the main under the street. Most Valley laterals were laid between 1946 and 1975, in vitrified clay or cast iron, and they have been sitting in dry soil for most of their lives. A wet winter puts them under a kind of load they rarely see.
Why rain ends up in a pipe built only for sewage
The sewer and the storm drain are supposed to be two separate systems. Rain from your roof goes to the gutter at the curb, then to a catch basin, then to the Los Angeles River channel and out to the ocean at Long Beach without any treatment. Sewage goes from your lateral into the city main and on to the Hyperion plant in Playa del Rey or the Tillman plant next to the Sepulveda Basin. The two are not meant to mix.
In practice they mix all the time, and engineers have a name for it: inflow and infiltration. Inflow is rain that runs straight in through an opening, like a cleanout with a missing cap at the side of the house or an old downspout someone tied into the line years ago. Infiltration is groundwater that seeps in through cracked clay, open joints and the gaps that roots pry open. In a dry month neither one matters. After three days of an atmospheric river, the soil around a 4-inch clay lateral is saturated and the water pushes in at every weak joint along its length.
Your lateral is only part of the problem. The 8-inch main under the street takes the extra water from every house on the block at the same time. When the main runs full, the sewage from your house has nowhere to go, and it comes back up at the lowest open drain in the house. In a raised-foundation home in Northridge, that is usually a shower or a floor drain. In a slab home in Van Nuys it can be the tub.
What the last strong El Niño winters did
Downtown Los Angeles averages 14.25 inches of rain a season. In the 1982-83 El Niño it got 31.25 inches. In 1997-98 it got 31.01, much of it in February 1998. Those two winters are why older plumbers in the Valley still say the word El Niño with a certain tone. Long runs of storms soaked the ground for weeks at a time, and that is the condition that turns a cracked lateral into a backup.
The 2015-16 El Niño was also rated strong, and downtown got 9.65 inches. Southern California stayed dry while the storms went north. So the forecast is about odds, and the odds this year favor rain. A lateral that is going to fail will fail in one good storm whether the season total ends up at 12 inches or 30.
The pipes most likely to give you trouble
We see the same patterns year after year in the Valley:
- Clay laterals with root entry. Ficus, elm and the big street trees planted along Valley parkways decades ago send roots into clay joints. Every joint a root opens is also an open door for groundwater.
- Bellied lines on flat West Valley lots. Winnetka, Canoga Park and Reseda sit on flat alluvial ground, and many laterals were laid at the minimum quarter inch of fall per foot. A small sag holds water and solids, and a storm surge in the main stops it from draining.
- Cast iron that has rusted through on the bottom. Cast iron under 1950s and 1960s slabs rots along the bottom of the pipe first. Once the bottom is open, the pipe behaves like a gutter, and soaked soil washes in.
- Lines that crossed the 1994 earthquake. The Northridge earthquake cracked and offset a lot of buried pipe, and some of it has never been looked at since. An offset joint is a ledge for debris and a gap for water.
Five things to check before the first storm
None of these take long. Doing them in November means any repair is a dry-ground job on a day you pick.
- Find your cleanout and make sure it has a cap. It is usually a 3 or 4-inch pipe sticking up near the foundation on the side of the house facing the street. A missing or cracked cap lets yard water pour straight into the sewer. A new cap costs a few dollars.
- Walk your downspouts. Each one should end in a splash block, a yard drain or a pipe that runs to the curb. If one disappears into the ground near the cleanout, it may be tied into the sewer. We cover that in more detail in our post on downspouts tied into the sewer.
- Think back to last winter. A toilet that gurgled during rain, a shower that drained slowly only in storms, or a sewer smell after rain all point to a lateral taking in water.
- Get a camera run if the line has not been seen in five years. A sewer camera inspection shows every crack, root and open joint, and the locator marks the depth on the surface so any repair is a known spot.
- Ask whether a backwater valve makes sense. If the house sits low compared to the street, a backwater valve is the one fitting that stops a full main from pushing sewage back into the house.
What we do in the weeks before the rain
Our work before a wet winter is mostly cameras and cleaning. We run the camera from the cleanout to the main, mark anything that will take in water, and clear roots and buildup with a cable or a hydrojetter depending on the pipe. If a section is broken, the dry months are when a spot repair or a trenchless liner can be done without pumping water out of the trench. A repair done in December costs less and goes faster than the same repair done during a storm with the house backed up.
If your line gave you trouble last winter, or if you just bought a house in the Valley and have no idea what is under the yard, call before the forecast turns into rain. Related reading: the Valley's El Niño history and the drains it left behind, and our FAQ.
Call BBC Rooter & Plumbing at 818-280-9135, Northridge, CA. CSLB #720343. Open 7 days, 6am to 6pm.
Frequently asked questions
Can rain really back up my sewer if my gutters are not connected to it?
Yes. Rainwater gets into a sewer two ways. It runs in through openings, like a missing cleanout cap or a downspout tied into the line, and it seeps in through cracks and open joints once the soil around the pipe is soaked. A clay lateral from the 1950s with a few root-opened joints can take on a lot of groundwater in a long storm. The city main under your street fills up with the same water from every house on the block, and when the main runs full, your lateral has nowhere to drain.
When will the El Niño rain actually start in Los Angeles?
Nobody can give a date. NOAA's winter outlook covers December through February and favors wetter than average conditions across the southern tier of the country, including California. In past strong El Niño years the heaviest Los Angeles storms came in January and February. The safe plan is to have any sewer work done before Christmas, while the ground is dry and a dig is still a scheduled job.
Is El Niño guaranteed to bring a wet winter?
No. The winter of 2015-16 was a strong El Niño and downtown Los Angeles got 9.65 inches of rain, well under the 14.25 inch normal. The odds favor a wet winter this time, but odds are not a promise. The checks in this article are worth doing in any year, because one big storm is enough to find a weak lateral.
How do I know if my sewer line takes in water when it rains?
The giveaways are a toilet that gurgles or drains slowly only during or right after rain, a floor drain or shower that backs up in a storm, and a sewer smell that shows up after rain. A camera inspection during dry weather will show the cracks, open joints and root entry points where the water gets in.
Should I install a backwater valve before El Niño?
If your lowest drain sits close to the level of the street, or if you have had a backup during a storm before, a backwater valve is the most direct protection there is. It lets your waste flow out and closes when the city main pushes water back toward the house. It needs a cleanout and a dig at the right spot on the lateral, so it is work for the dry weeks, not for the night of the storm.